In recent years, thermal transfer systems have been developed to obtain prints from pictures, which have been generated electronically from a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with an element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to one of the cyan, magenta or yellow signals, and the process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271.
Dye-receiving elements used in thermal dye transfer generally include a support (transparent or reflective) bearing on one side thereof a dye image-receiving layer, and optionally additional layers. The layer comprises a polymeric material chosen from a wide assortment of compositions and should have good affinity for the dye. Dyes must migrate rapidly into the layer during the transfer step and become immobile and stable in the viewing environment. One way to immobilize the dye in the receiving element is to transfer a laminate layer from the donor element to the receiver after the image has been generated. The layer must also not stick to the hot donor during the printing process, otherwise the final image will be damaged due to either the donor or receiver tearing while peeling apart after the printing step. One way to prevent donor-receiver sticking is to apply an overcoat layer or to add release agents to the receiver layer. The overcoat would require a separate coating step which increases manufacturing costs of the element and addition of release agents increases the media costs.
U.S. Pat. No. 5,317,001 relates to thermal dye transfer to a receiver element. The layer is described as comprising a water-dispersible polyester. These materials are aqueous coatable and were found to provide good image-receiving layer polymers because of their effective dye-compatibility and receptivity. However, there is a problem with this material in that severe donor-receiver sticking occurs during the printing process.
U.S. Pat. No. 5,427,847 relates to the wax transfer of dyes to a receptor sheet. The receptor sheet comprises a mixture of a wax coating having a Tg below 25 degrees C. and a polymeric material which is used in a wax transfer process, and not a thermal dye transfer process. In addition, the weight ratio of wax to polymer is described to be from 2:1 to 12:1, whereas the amount of wax in the receiving layer of the present invention is relatively small.
Polycarbonates (the term “polycarbonate” as used herein means a carbonic acid and a diol or diphenol) and polyesters have been suggested for use in image-receiving layers. Polycarbonates (such as those disclosed in U.S. Pat. Nos. 4,740,497 and 4,927,803) have been found to possess good dye uptake properties and desirable low fade properties when used for thermal dye transfer. As set forth in U.S. Pat. No. 4,695,286, bisphenol-A polycarbonates of number average molecular weights of at least about 25,000 have been found to be especially desirable in that they also minimize surface deformation that may occur during thermal printing.
U.S. Pat. No. 5,317,001 (Daly et al.) describes a thermal dye transfer receiving element utilizing an aqueous dispersible polyester. While the dye transfer layer in Daly et al does result in an excellent image, the use of problematic lubrication is required to prevent dye donor element sticking to the dye-receiving layer.
Polymers may be blended for use in the layer in order to obtain the advantages of the individual polymers and optimize the combined effects. For example, relatively inexpensive unmodified bisphenol-A polycarbonates of the type described in U.S. Pat. No. 4,695,286 may be blended with the modified polycarbonates of the type described in U.S. Pat. No. 4,927,803 in order to obtain a receiving layer of intermediate cost having both improved resistance to surface deformation which may occur during thermal printing and to light fading which may occur after printing. A problem with such polymer blends, however, results if the polymers are not completely miscible with each other, as such blends may exhibit a certain amount of haze. While haze is generally undesirable, it is especially detrimental for transparent labels. Blends that are not completely compatible may also result in variable dye uptake, poorer image stability, and variable sticking to dye donors.